In a quench hardening treatment of an annular member such as a bearing ring, in order to suppress deformation during heat treatment (heat treatment deformation) or poor circularity, restrained quenching in which cooling in quenching is performed while the annular member is restrained may be adopted in some cases. This restrained quenching utilizes expansion of steel forming the annular member through martensitic transformation during quenching. Namely, cooling in quenching is performed while the annular member is surrounded by a restraint member so that the annular member expands along a wall surface of the restraint member and the annular member in a desired shape can be obtained. According to this method, however, an inner wall of the restraint member and the annular member come in intimate contact with each other at the time point when cooling in restrained quenching ends. Then, it is difficult to separate the annular member from the restraint member, and efficiency of the quench hardening treatment may lower.
In order to address this, a restrained-quenching method has been proposed, in which a restraint member having annular openings in upper and lower portions and having an inner wall like a cylinder is adopted, the annular member is successively pushed in from the upper opening, the annular member is cooled, and the cooled annular member is pushed out of the lower opening. Thus, the annular member is successively separated from the restraint member and lowering in efficiency of the quench hardening treatment can be suppressed (Japanese Patent Laying-Open No. 9-176740 (Patent Document 1)).    Patent Document 1: Japanese Patent Laying-Open No. 9-176740